Tree harvesting apparatus, such as described in U.S. Pat. No. 3,690,354 - Moser, and which is hereby incorporated by reference, are effectively used for cutting down a tree, positioning the severed tree with the trunk oriented generally horizontally, and further processing the tree by, for example, severing limbs of the tree from the trunk.
During cutting down and positioning the tree, it is necessary for the harvesting apparatus to be tightly secured to the tree in order to maintain control over the severed tree. The tree is forcibly maintained relative to the harvester by a grapple which is positioned about the tree trunk and forcibly engaging said trunk.
During further processing of the tree, subsequent to orienting the severed tree generally horizontally, it is desirable to have improved control over the forcible engagement of the tree by the grapple. This invention thereby resides in method apparatus and a hydraulic system for controlling the tree engaging pressure of the grapple which results in improving the operation of the harvester during processing of the tree.